These days, to protect the environment and prevent global warming, the suppression of the consumption of fossil fuel is increasingly demanded, and the demand influences various manufacturing industries. For example, automobiles, which are indispensable to daily life and activity as a moving means, are no exception, and improvements in fuel efficiency etc. by the weight reduction of car bodies etc. are required. However, for automobiles, simply achieving a weight reduction of the car body is not permitted in terms of the functionality of the product, and it is necessary to ensure proper safety.
Most of the structure of the automobile is formed of iron-based materials, in particular steel sheets, and the reduction in the weight of the steel sheet is important to the weight reduction of the car body. However, as described above, simply reducing the weight of the steel sheet is not permitted, and ensuring the mechanical strength of the steel sheet is required at the same time. Such a demand on the steel sheet is placed not only in the automobile manufacturing industry but also in various manufacturing industries similarly. Hence, research and development are being made to enhance the mechanical strength of the steel sheet and thereby obtain a steel sheet in which the mechanical strength can be maintained or improved even when the wall thickness is made smaller than those of conventionally used steel sheets.
In general, a material having high mechanical strength tends to decrease in shape fixability in molding such as bending, and is difficult to mold into a complicated shape. As a means for solving such a problem with moldability, what is called “the hot pressing method (also called the hot stamping method or the die quenching method)” is given. In the hot pressing method, a material to be molded is once heated to high temperature, the steel sheet softened by heating is pressed to be molded, and then cooling is performed. By the hot pressing method, the material of the object can be easily pressed because the material is once heated to high temperature and softened. Furthermore, the mechanical strength of the material can be enhanced by the quenching effect by the cooling after molding. Thus, a molded product in which both good shape fixability and high mechanical strength are achieved can be obtained by the hot pressing method.
However, when the hot pressing method is used for a steel sheet, the surface of the steel sheet is oxidized by the steel sheet being heated to a high temperature of 800° C. or more, and scales (compounds) are produced. Hence, the process of removing the scales (what is called a descaling process) is needed after hot pressing is performed, and productivity is reduced. In addition, in a member etc. requiring corrosion resistance, it is necessary to perform anti-rust treatment or metal covering on the surface of the member after processing, and a surface cleaning process and a surface treatment process are needed; consequently, productivity is further reduced.
As a method to suppress such a reduction in productivity, for example, a method in which a steel sheet to be hot pressed is provided with a covering in advance is given. Various materials such as organic-based materials and inorganic-based materials are generally used as the covering on the steel sheet. Among these, plated steel sheets based on zinc (Zn), which has a sacrificial anti-corrosion action on the steel sheet, are widely used as automotive steel sheets etc. from the viewpoints of the anti-corrosion capacity and the steel sheet production technique.
By providing a Zn-based metal covering, the production of scales on the surface of the steel sheet can be prevented, and processes such as descaling become unnecessary; thus, the productivity of molded products is improved. In addition, the Zn-based metal covering has also an anti-rust effect, and therefore also corrosion resistance is improved. Patent Literature 1 to Patent Literature 4 below disclose a method of hot pressing a plated steel sheet that is obtained by providing a Zn-based metal covering to a steel sheet having a prescribed component composition.
In Patent Literature 1 to Patent Literature 3 below, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet is used as a steel sheet for hot pressing. By using a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet for hot pressing, a structure member can be molded without iron oxides (that is, scales) being formed on the surface. Further, in view of the fact that, when a Zn oxide layer is formed thick on the surface of a heat-treated steel material obtained by hot pressing a Zn-based plated steel sheet, the coating film adhesiveness and the post-coating corrosion resistance of the heat-treated steel material are adversely affected, Patent Literature 4 below discloses an invention in which a heat-treated steel material is subjected to shot blasting to remove a Zn oxide layer or is subjected to coating after the thickness of a Zn oxide layer is reduced.
Patent Literature 5 and Patent Literature 6 below disclose an invention that improves the coating film adhesiveness and the post-coating corrosion resistance of a heat-treated steel material obtained by hot pressing a Zn-based plated steel sheet. Patent Literature 5 below discloses an invention in which a hot-dip galvanized steel sheet with its surface covered with a silicone resin coating film is used as a steel sheet for hot pressing, and Patent Literature 6 below discloses an invention in which a hot-dip galvanized steel sheet covered with a barrier layer containing phosphorus (P) and silicon (Si) (a phosphate is given as an example of P, and colloidal silica is given as an example of Si) is used as a steel sheet for hot pressing.
Patent Literature 7 below discloses a technology in which elements that are easier to oxidize than Zn (easily oxidizable elements) are added into a galvanized layer and an oxide layer of these easily oxidizable elements is formed on the outer layer of the galvanized layer during the temperature increase in hot pressing, and thereby the volatilization of Zn is prevented.
According to the inventions disclosed by Patent Literature 5 to Patent Literature 7 below, since a galvanized layer is covered with the barrier layer described above, the vaporization of Zn is suppressed, and thus the adhesiveness of an intermediate coating film and an over-coating film and post-coating corrosion resistance are good.